1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method in which an image is formed on a recording medium by using an ink and a treatment agent.
2. Description of the Related Art
In the inkjet recording system, the recording is performed by ejecting and depositing ink droplets onto a recording medium from a plurality of nozzles formed in an inkjet head. The inkjet recording system is able to record images of high resolution and high quality, with little noise during the recording operation and low running costs. The ink ejection method can be, for example, a piezoelectric method, which uses the displacement of a piezoelectric element, or a thermal method, which uses thermal energy generated by a heating element, or the like.
In the inkjet recording system, dots (hereinafter referred to as “ink dots”) formed of ink droplets deposited on the recording medium produce bleeding and hence there is a problem in that the printing properties decline markedly. In order to solve this problem, there is widely known technology which previously deposits a liquid (treatment liquid) containing a component that reacts with the coloring material (pigment or dye) in the ink and causes the coloring material to aggregate or increases the viscosity of the ink itself, and subsequently deposits ink droplets. The methods for depositing the treatment liquid can be divided broadly into methods that apply the treatment liquid by using an application roller, or the like, and methods that eject droplets of the treatment liquid by using an inkjet head. The latter type of method is preferable since it allows the droplets of the treatment liquid to be deposited selectively at the positions where the ink droplets are to be deposited (the ink droplet deposition positions).
However, if droplets of the treatment liquid are deposited by means of the inkjet recording system, then dots (hereinafter referred to as “treatment liquid dots”) formed of the treatment liquid droplets that are deposited onto the recording medium combine with each other on the surface of the recording medium, and hence there arises a problem of non-uniformity of the treatment liquid on the recording medium. If non-uniformity arises in the treatment liquid, then the reaction with the ink becomes non-uniform and this gives rise to decline in image quality.
Japanese Patent Application Publication No. 2004-142291 discloses technology for a method of forming images on a recording medium by using ink (a first liquid composition) and a treatment liquid (a second liquid composition), wherein the surface tension of the treatment liquid is increased and the treatment liquid droplets are deposited in a thinned out fashion in such a manner that the respective dots of the treatment liquid do not make contact with each other on the recording medium. Nevertheless, this involves the following problems (1) to (3).
(1) When droplets of the treatment liquid are deposited in a thinned out fashion, then the amount of the reactive agent (treatment liquid) is reduced and hence there is a problem in that the reactivity with the ink becomes worse. For example, if a line is printed in an oblique direction, then droplets of the treatment liquid are not deposited completely onto the pixels that correspond to the edge portions of the line, and hence there is severe bleeding of the line.
FIG. 14 is an illustrative diagram showing an example of the droplet deposition positions of the treatment liquid and the ink in the related art. Here, a case is described in which the droplet deposition density of the treatment liquid is thinned out at a rate of 1/2 with respect to the droplet deposition density of the ink (for example, the droplet deposition density of the ink is set to 1200 dpi and the droplet deposition density of the treatment liquid is set to 600 dpi). In other words, if all of the pixels shown in FIG. 14 are set as droplet deposition positions for the ink droplets, then the pixels applied with the hatching (the hatched pixels) are the droplet deposition positions of the treatment liquid droplets. Here, it is supposed that treatment liquid droplets are deposited in a case where ink droplet deposition data (ink dot data) is present at the droplet deposition positions of the treatment liquid droplets (hatched pixels) set as described above. Under these droplet deposition conditions, if a line is formed in an oblique direction as shown in FIG. 14, then although the treatment liquid droplets are deposited at the positions of the ink dots 900 in the central portion of the line (the solid circles in the drawing), no treatment liquid droplets are deposited at the positions of the ink dots 902 in the edge portions of the line (the hollow circles in the drawing). Consequently, it is not possible for the ink droplets that have been deposited on the edge portions of the line to react with the treatment liquid droplets, and hence, the edge portions of the line suffer bleeding and the image quality declines.
If treatment liquid droplets are simply deposited in a thinned out fashion in this way, then problems may occur, depending on the image. In order to avoid this, for example, edge portions must be extracted when creating the dot data for the treatment liquid, and hence the load on the system increases.
(2) The surface tension of the treatment liquid is made greater than the surface tension of the ink, but if the treatment liquid and the ink have different surface tensions, then it is necessary to prepare respective waveforms for driving actuators to eject droplets of the treatment liquid and the ink, and hence the system becomes more complicated and costs increase.
(3) As shown in FIG. 15, if a layer of treatment liquid (treatment liquid layer) 912 is present on the recording medium 910, then the droplets of ink 914 that have been deposited float about in the treatment liquid layer 912, thus causing the ink coloring material (ink dots) 916 to move, and as a result, there is a problem in that the output image is greatly disturbed in comparison with the desired image. The problems described above are especially grave in cases where a recording medium having a surface coating (for example, coated printing paper, or art paper, etc.) is used, or cases where the time period (interval) from the deposition of the treatment liquid droplets until the deposition of the ink droplets is shorter than the permeation time of the treatment liquid into the recording medium.